This invention window assembly has general applicability to any type of apparatus which requires the introduction of high power microwave energy from a source such as a waveguide or antenna, maintained at substantially atmospheric pressure, into the interior of a vacuum chamber, maintained at subatmospheric pressure. The microwave energy is introduced into the vacuum chamber for effecting a glow discharge plasma which is utilized to either deposit a semiconductor or insulating material onto the exposed surface of a substrate or to remove (etch) material from that exposed surface. Whereas the invention window assembly has universal applicability to microwave apparatus, the invention window assembly is especially applicable to the fabrication of photo responsive alloys and devices for various photoconductive applications including the fabrication of electrophotographic photo receptors. Alternatively, the invention window assembly may be employed with equal advantage in association with a vacuum chamber adapted to etch or otherwise treat or modify the surface of a substrate.
Regardless of the type of microwave plasma operation (deposition or etch) being conducted, the rate at which that operation occurs can be controlled, inter alia, by controlling the power at which the microwave energy is transmitted into the interior of the vacuum chamber. In order to deposit or etch at a high rate, it is necessary to utilize high power levels, for example in the kilowatt range and preferably three or more kilowatts. However, the use of such high power microwave energy tends to cause heating of the dielectric window through which the microwave energy is coupled into the interior of the vacuum chamber, and prolonged or excessive heating of the dielectric window can cause cracking of the window with resultant catastrophic failure of the deposition/etch operation. Further, even the introduction of relatively low microwave power into the vacuum chamber over a relatively lengthy period of time can also cause the dielectric window to overheat and fail.
In an effort to overcome failure of the dielectric window due to overheating, it has previously been proposed to position a second window rearwardly of the window in the vacuum chamber wall and pass a cooling fluid between the two windows so as to reduce the temperature of the window positioned in the wall of the vacuum chamber to an acceptable level to allow the introduction of high power microwave energy into the vacuum chamber through the window without producing failure of the window even over extending periods of operation.
However, the spaced dual window arrangement creates problems with respect to coupling the microwave energy into the vacuum chamber since the waveguide surface transmitting the microwave energy from the microwave propagating means extends only to the rear or outboard surface of the second window so that the microwave energy thereafter moves in an uncontrolled manner into the vacuum chamber with the result that the shape and dimensions of the microwave energy in the space between the rear surface of the second window and the vacuum chamber become promiscuous and uncontrolled with the result that the microwave energy spreads out as it enters the vacuum chamber. This promiscuous spreading and deterioration of the form and dimensions of the microwave energy substantially derogates the efficiency of the deposition or etching operation taking place within the vacuum chamber and also severely complicates the task of providing a seal as between the waveguide surface and the cooling fluid circulating between the spaced windows since the randomly and promiscuously moving microwave energy will attack and ultimately destroy anything other than very expensive and very exotic seal arrangements.
More specifically, if an elastomeric or O-ring type seal is employed to seal the cooling fluid from the interior of the waveguide, the promiscuous microwave energy moving between the rear surface of the second window and the vacuum chamber causes a capacitive effect to develop in the vicinity of the elastomeric seal and the discharge activity resulting from the capacitive build-up interferes with the deposition/etching process and also derogates the elastomeric seal.
Accordingly, a need exists for an improved and inexpensive window assembly which can efficiently, economically, reliably and safely transmit relatively high power microwave energy from a waveguide into a vacuum chamber even over extended periods of use.